Cryptococcosis is a life-threatening illness that occurs at high frequency in individuals with AIDS. The therapy of cryptococcosis in patients with AIDS is unsatisfactory because the disease is associated with high mortality, and even when controlled with medication, C. neoformans infections are seldom eradicable in the setting of severe immune suppression. In the past decade, this research program has supported the scientific development of a novel therapy for cryptococcosis based on administration of a protective antibody. Passive antibody therapy is currently in clinical evaluation. It is a central tenet of immunological dogma that the specificity of an antibody (Ab) for an antigen is determined solely by interactions between the epitope in the antigen and the Ab variable region. In the past funding period, we have learned that, for some Abs, the constant region can also affect specificity. If confirmed, this observation has the potential to be a major discovery in immunology because it could influence our understanding of such fundamental concepts as the origin of the secondary immune response, immunological memory, and the basis of Ab specificity. Furthermore, this finding is of great importance for the future development of Ab therapies against cryptococcosis and other conditions. This application proposes to obtain unequivocal proof for the existence of this effect and to elucidate the mechanism by which the constant region can affect specificity. Four specific Aims are proposed: 1. To determine how antibody specificity for C. neoformans polysaccharide depends on isotype; 2. To determine the mechanism by which constant region structural motifs influence antibody specificity for C. neoformans polysaccharide; 3. To establish the contribution of constant region glycosylation to specificity and complete the characterization of 18B7 glycosylation; and 4. To validate the findings made with murine mAbs on human C regions. It is anticipated that this research program will produce new insights into Ab structure and function that are relevant for Ab-based strategies against C. neoformans and also to other efforts to develop Ab therapy, vaccines and fundamental immunology. [unreadable] [unreadable] [unreadable]